Up until the last round of "Balancing" Arx earning, exploration earned very few arx, better if you SRV'd a bit and flew out to map lots, but at around 30 mill data you could go from 0 to hero...
Now more Arx are earned while exploring, but less given for data, so I think around 50 mill is needed now. If I'm away on a jaunt, I usually plan to return Weds...
Edit: It doesn't advise you of the amount needed, so you could sell your 540 mill in one fell swoop and max out - I just did a little experimenting with the figures initially, and again after the change.
I'll sell in batches and exit to the main menu to check progress as i sell.
This could be quite frustrating though.